Presented
at the DesignCon98

Abstract

An innovative distributed termination scheme has been
developed for GTL+ bus designs which addresses reduction of reflections
under partial module loading conditions and solves the problem of bus
termination voltage generation in passive backplane 48 volt powered
systems. It has been found that for specific but realistic logic card
loadings conventional parallel termination results in small receiver noise
margins due to surprisingly large reflections between the load terminations
and the unpopulated portions of the bus. Additionally, in 48 volt powered
systems where the high to low DC-DC regulation takes place on each logic
module, the provision of termination voltages using a DC-DC converter on
the backplane violates the requirement for an entirely "passive"
backplane. The termination voltage can be more conveniently provided from
one or more of the logic modules.

For these reasons, NESA engineering staff has developed an
unconventional distributed termination combining end point termination,
with its desirable feature of reduced reflections in a fully loaded system,
with additional on-card distributed terminations to reduce reflections for
partial load configurations. This termination scheme is called
"combined or distributed termination".

This paper will compare the cases of conventional end point terminations
with the distributed "combined termination" to demonstrate the
advantages of this new termination scheme in terms of noise margin and
reduced bus reflections under partial and full loading in a 100 MHz GTL bus
system. Generation of on-card termination voltages simplifies the
requirements for a passive 48 volt backplane system.

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